In the fabrication of semiconductor devices, the surface of the semiconductor material in which the devices are fabricated must be substantially free of both physical and crystalline defects. A high degree of crystalline perfection is necessary to produce reliable devices having good electrical properties. In order to control the properties of such devices, it is necessary to be able to determine the quality of semiconductor material that is being used to make the devices.
Defects in semiconductor material (e.g., silicon, InP, InGaAsP or the like) include dislocations, stacking faults, oxygen precipitates and heavy metal precipitates. Such defects, which are often one to two microns in size, have been detected using a variety of techniques including etch pit analysis, x-ray topography and electron beam induced current.
Etch pit analysis involves etching a silicon wafer and then viewing the pits that grow in the defective areas under an interference microscope. Individual stacking faults, dislocations and saucer pits can be distinguished after etching by the shape of the pits that each defect causes to grow. X-ray topography is accomplished by setting up an x-ray beam, the wafer and a film to catch Bragg-reflected rays. The relative angular positions of these three components are such that the Bragg law is satisfied when the beam strikes good material, but is not satisfied when it encounters defects. The film and sample are translated such that the reflected beam intensity for all points on the wafer are mapped onto the film. Spatial resolutions of 1-10 microns can be attained after 3 to 20 hours of exposure.
Electron beam induced current (EBIC) is accomplished inside a scanning electron microscope (SEM). The SEM beam is used to induce carriers within a sample, which are then separated by an electric field. The field is created by either a pn junction within the sample, a Schottky junction on the surface of the sample, or by mounting the sample within an external field. Contact to the sample is generally accomplished with silver paste or spring loaded fine wires. As the SEM beam is rastered in x-y plane the collected carriers provide intensity modulation of an x-y display. Carrier recombination at defects allows them to appear dark on the display screen, down to a spatial resolution of 0.5 micron. Although these techniques image defects with high spatial resolution in semiconductor material, they are either destructive, time consuming and/or require a vacuum.
One non-destructive optical technique for determining electrical non-uniformities in semiconductor wafers is described in U.S. Pat. No. 4,211,488 to Kleinknecht which issued on July 8, 1980. That patent makes use of the fact that crystalline imperfections or doping striations in a semiconductor wafer cause lower carrier lifetime and/or mobility during photoexcitation and therefore change the infrared reflectance of the material. The electrical non-uniformities or defective areas are detected by irradiating an area of the semiconductor wafer with a beam of monochromatic light having energy greater than the bandgap energy of the semiconductor wafer material. This will photoexcite (i.e., pump) a high density of electrons and holes which changes the infrared reflectance at the pumped area. The same surface area of the wafer is simultaneously irradiated with a second beam of monochromatic light having an energy less than the bandgap of the semiconductor material, whereby part of the second beam is reflected from the surface.
If the monitored area has moderate to low defect density and high carrier mobility, the reflectance of the surface will change during photoexcitation and the intensity of the reflected second beam will also change. However, if there is a high defect density within the area the reflectance of the surface will not change during photoexcitation and the intensity of the reflected second beam will remain unchanged. The intensity of the reflected beam is detected and the magnitude thereof is a measure of the carrier mobility and recombination time which is directly related to the density of the surface or near surface defects in the semiconductor material. The light beams in the Kleinknecht patent simultaneously illuminate an area of about 0.25 square mm.
Although such a technique can effectively provide information as to the average carrier lifetime and mobility over the 0.25 square mm area, it cannot resolve individual defects of one to two microns in size. There are two fundamental reasons for this fact. First, the laser providing the below-bandgap energy emits long wavelength light in the infrared part of the spectrum. Since basic diffraction theory predicts that minimum obtainable spot size is proportional to the f number times the wavelength, infrared light having wavelengths of interest for defect detection can be focused to spots no smaller than 10 to 20 microns. Second, the probe beam in the Kleinknecht patent has a high angle of incidence with respect to a normal to the wafer surface. This high angle leads to a further enlargement of the probe beam spot. Therefore, such a technique cannot focus the infrared beam to a small enough spot to resolve individual defects, of micron size, due to accepted basic optical theory. However, there is clearly great interest in resolving these micron-sized defects due to their influence on VLSI circuits having micron-sized features.
Accordingly, there is a need for a non-destructive defect detection system in which individual defects of one to two microns in size can be resolved.